MEMORANDUM 

From: The U.S. Environmental Protection Agency 

To: EGU NSPS Docket (EPA-HQ-OAR-2013-0495) 

Date: September 2013 

Subject: Standard of Performance for Natural Gas-Fired Combustion Turbines 

Summary 

To establish an appropriate, combustion turbine-based standard, we reviewed the emissions rates of natural gasfired (non-CHP) combined cycle units used in the power sector that commenced operation on or after January 1, 

2000 and that reported complete generation data to the EPA. Based on this analysis, over 96% of these units 

achieve the proposed standards with no excess emissions. 

Abbreviations 

CHP combined heat and power 

CO2 carbon dioxide 

EGU electric generating unit 

EPA (U.S.) Environmental Protection Agency 

MMBtu million British thermal units (unit of energy) 

MW megawatt(s) - one million watts

MWh megawatt hours(s) 

NGCC natural gas combined cycle

Introduction 

The purpose of this study is to determine how existing natural gas1

-fired combined cycle (NGCC) units have 

performed in comparison to the proposed standards. The study is based on hourly emission and operation data 

reported to the Environmental Protection Agency (EPA). Approximately 4,700 electricity generating units2 are 

applicable to the Acid Rain or Clean Air Interstate Rule programs. Of these, some 1,000 are NGCC located in 

41 states and encompass a diverse population in capacity, years of service and configuration. This study also 

explores whether factors such as cycling, ambient temperature, or elevation affect emission rates. NGCC 

technology performance and efficiency has improved over time.3 Since this study is retrospective, i.e., 

measuring the performance of units that are less efficient than currently available on the market, it is inherently 

conservative. 

The term ‘unit’ here refers to the natural gas-fired combustion turbine, heat recovery steam generator and duct 

burner (if any) together in various configurations. They fall into two groups: single-shaft, where the turbine and 

heat recovery steam generator are in line, and multiple-shaft, where two or more turbines may be harnessed to a 

single steam generator. In either case, for this analysis both are counted as a single NGCC unit. 

The analysis covers monthly emission and operation data for 2007 to 2011. Not all units ran for all 60 months 

so the total of ‘unit operating months’ used in this analysis is 15,670 out of 18,420 possible total months. The 

study includes only those months where both CO2 mass and gross MWh output were greater than zero. All 

                                                           

1 Natural gas refers to the primary fuel as reported to the EPA and includes both ‘Natural Gas’ and ‘Pipeline Natural Gas’. ‘Process

Gas’ and ‘Other Gas’ units, typically associated with industrial production, are excluded.

2 An electricity generating unit may be referred to as a ‘facility’ in NSPS.   3 http://www.westgov.org/wieb/electric/Transmission%20Protocol/SSG‐WI/pnw_5pp_02.pdf  See vintage heat rate improvement;

the annual average is assumed to improve ‐0.6% per year over 2002‐25.
emission and operation data used in this analysis are publically available for download, including unit megawatt 

capacity, fuel type and monthly operating time, CO2 mass and gross load in megawatt hours (MWh).4

 The 

publically available data is essentially the same data that would be used for compliance determinations since 

there is no bias adjustment in Appendix G of Part 755 for units using fuel meters to determine CO2 emissions. 

The unit population is filtered to include only those NGCC units that report natural gas or pipeline natural gas 

as primary fuel, are in the electric utility or small power producer source categories and have commercial 

operation dates on or after January 1, 2000. In addition, the study is limited to those units that reported gross 

output data from both the primary combustion turbine and steam generator during the study period.6

 The study 

excludes all units at plants where cogeneration is reported.7

 

The resulting population of 307 NGCC units is heterogeneous in location, age, capacity and operating profile. 

They are located in 33 states and all 10 EPA regions as shown in Table 1. Both large and small states are 

represented, including California with 43 units and Rhode Island with 3. Table 2 summarizes the study 

population by the year they entered service between 2000 and 2010. Although new NGCC units came online in 

2011, none are included here as they lacked sufficient data to calculate a 12-month rolling average, the basis for 

determining the standard of performance. The average study unit has been in service for over 8 years. The study 

population includes 11 units that our research determined were retrofits or conversions of simple cycle turbines 

to NGCC or that operated in single cycle mode for a period of time during commencement of operations. 

Table 1: Study Units by EPA region and State 

State EPA 

Region 

Unit 

Count 

MW 

capacity 

AL 4 20 6,613 

AR 6 20 4,854 

AZ 9 22 7,271 

CA 9 43 11,818 

CO 8 5 1,107 

CT 1 2 590 

FL 4 20 7,625 

GA 4 8 2,840 

ID 10 1 314 

IL 5 8 2,454 

IN 5 2 690 

LA 6 7 2,225 

MA 1 2 858 

ME 1 3 906 

MI 5 3 1,206 

MN 5 6 1,993 

MO 7 2 568 

MS 4 14 4,773 

NE 7 2 290 

NH 1 2 580 

NM 6 5 1,310 

NV 9 8 2,584 

Table 2: Study population by year commercial 

operation commenced 

Online Year Unit 

Count 

MW 

Capacity 

2000 22 5,994 

2001 33 10,331 

2002 71 20,471 

2003 64 21,817 

2004 20 6,158 

2005 39 11,300 

2006 9 3,131 

2007 6 1,605 

2008 26 9,062 

2009 13 5,165 

2010 4 1,315 

                                                           

4 Data is available at http://ampd.epa.gov/ampd/ 5 See part 75 in Volume 40 of the Code of Federal Regulations (CFR) originally published in January, 1993.   6 Reporting output from the steam generator is optional. See 40 CFR part 75. 7 Excludes units at plants with any cogeneration or CHP noted in eGRID, Greenhouse Gas Reporting Program, CAMD, Ventyx (EIA) or

NEEDs databases.  
NY 2 9 3,006 

OH 5 6 2,100 

OK 6 17 5,723 

PA 3 10 3,508 

RI 1 3 921 

TN 4 2 655 

TX 6 33 9,893 

UT 8 5 1,341 

VA 3 5 1,652 

WA 10 4 1,282 

WI 5 8 2,799 

In capacity, the units range from 60 to 451 MW with an average of 314 MW. The mean annual capacity factor 

during the study period was 36 percent, but ranged from a low of 33 percent in 2008 to a high of 38 percent in 

2010 when annual generation peaked. Between 2007 and 2011, the annual CO2 rate, expressed in pounds of 

CO2 per megawatt hour of gross output (lb CO2/MWh), was consistently in the range of 845 to 848 lb 

CO2/MWh, while the count of operating units grew from 264 to 307. 

Level of the Standard

The proposed rule establishes two subcategories for NGCC. Small units, with a gas turbine heat input capacity 

less than or equal to 850 MMBtu (approximately 70 MW8), are required to meet an output-based performance 

standard of 1,100 lb CO2/MWh. Large NGCC units, with a heat input capacity greater than 850 MMBtu, are 

required to meet an output-based standard of 1,000 lb CO2/MWh. Note that output is measured as gross, not net, 

electrical output. Since the study does not include cogeneration units, it gives no consideration to steam output. 

Fourteen study units are in the small subcategory and 293 are in the large subcategory. As shown in Table 3, the 

small units average 70 MW capacity while the large unit mean is 325 MW. 

Table 3: Small and Large Subcategory Units 

Subcategory Unit Count 

Average 

Capacity 

(MW) 

Small 14 70 

Large 293 325 

All Units 307 314 

Calculation of the Standard 

Compliance with the standard in the proposed rule is determined by calculating the 12-operating month rolling 

average. To calculate compliance, a unit’s total CO2 emissions over the twelve previous operating months is 

divided by the unit’s total electrical energy output over that period. The formula is: 

A = C / M 

A is the 12-month rolling average (lb CO2/MWh). 

C is the sum of CO2 mass emissions for the past 12 operating months in pounds (lbs) 

M is the sum of gross output for the past 12 operating months in MWh 

                                                           

8 Capacity for combustion turbine engine only.   
A calculation result only becomes available once 12 months of operating data are available. Since the study 

period is 60 months long the maximum number of rolling average results is 49. An ‘exceedance,’ as used here, 

means that a unit exceeded the applicable subcategory standard of performance for one 12-month rolling 

average. Since the calculation only includes operating months, the actual time span covered may be more than a 

calendar year. For example, if a unit started operation in January 2007, but did not operate in March of that 

year, the first rolling average would not be available until January 2008. Unless stated otherwise, the term ‘CO2

rate’ as used here means the 12-month rolling average CO2 emission rate. 

There are a number of advantages to using this approach. A 12-month rolling average gives more weight to 

high-load hours and more accurately reflects overall environmental performance. In addition, because low-load 

hours do not factor as heavily into the calculated average, it reduces the impact of startup and shutdown periods 

when the unit may be operating less efficiently. A 12-month rolling average, as opposed to a calendar year 

average, uses 12 months of data and equitably addresses new and retiring units regardless of what month they 

begin or cease operation. 

Fleet Averages 

EPA calculated the 12-month rolling averages for this study using hourly unit emissions data aggregated to the 

monthly level. The key parameters examined are monthly CO2 mass (lbs), gross output (MWh), and operating 

time (hours). The calculation is based on operating months, i.e., months in which both CO2 mass emissions and 

gross output are greater than zero. The fleet average monthly CO2 rate9 for all study units is 861 lb CO2/MWh 

with a distribution (Figure 1) that is slightly right-tailed due to four units with high rates that exceed three 

standard deviations from the mean. The fleet average monthly CO2 rate for the small unit subcategory over the 

five-year study period is 907 lb CO2/MWh. There is a wide range in the monthly fleet rate among the small 

units, from a low of 858 (May, 2009) to a high of 993 (July, 2008) lb CO2/MWh. In contrast, the fleet average 

monthly rate for the large unit subcategory is 859 lb CO2/MWh and is consistent within a two percent range 

over the same period. 

Figure 1: Unit Average CO2 Rate Distribution 2007-2011

The 1,100 lb CO2 / MWh standard for the small unit subcategory is justified by both technological and 

operating characteristics. In general, smaller capacity NGCC units available on the market today are less 

efficient than the largest units. Manufacturers of the largest NGCC units now report achieving overall thermal 

efficiency of over 60 percent while the small units currently top out at approximately 54 percent.10 This 

                                                           

9 Fleet average means calculating the composite 12‐month rolling average for the defined group of units then averaging the resulting

48 months. 10 Compare, for example, Siemens H frame to General Electric LM6000.

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

600 700 800 900 1,000 1,100 1,200

Normal Distribution

Unit Averge 12‐Month Rolling CO2 Rate 2007‐2011
difference in efficiency is reflected in the average CO2 rates noted above: the monthly CO2 rate average for 

large units is 5% lower than the small units. The five-year average unit capacity factor for all units was 36 

percent. As shown on Table 4, small units operate at a lower average capacity factor, 27 percent. However, both 

categories include units operating across a wide range of capacities during the study period. The typical unit 

with a high capacity factor operated in 55 or more months (out of 60) in the study period.11 

Table 4: Small and Large Subcategory Capacity Factors (Percent) 

Subcategory 

Average 

Capacity 

Factor 

2007-2011 

Small units 27 

Large units 36 

All units 36 

Exceedances

Out of 307 units, over 92 percent (284) had no exceedances during the five year study period. The 23 units with 

exceedances included one in the small unit subcategory and range from 80 to 423 megawatt capacity. They are 

located at thirteen different facilities in ten states that span the country (AR, CA, FL, IL, MO, MS, NE, NY, 

TX, WA). Five units had a single exceedance; the remaining 18 units ranged from 2 to 39 exceedances each. 

The units with exceedances are a diverse group by location and capacity but share some characteristics. They 

report lower utilization factors and fewer operating months than the fleet average. In addition, the nineteen 

least-efficient units, i.e., those with highest average CO2 rates over the study period, all had one or more 

exceedances. This analysis is conservative since 12-month averages where electric sales were not sufficient for 

the facility to be considered an EGU (i.e., selling one-third or more of its potential electric output and more than 

219,000 MWh net-electric output to a utility distribution system) were not filtered out of the analysis. 

Temperature 

Combustion turbines may operate less efficiently at higher ambient temperatures, however, a comparison of 

winter monthly (December, January and February) to summer monthly (June, July, August) rolling average CO2

rates in this study found no difference overall in the NGCC CO2 emission rate. Both time periods had nearly 

identical fleet average CO2 rates of 864 lb CO2/MWh. Almost half (144) of 301 units operating in both time 

periods, had lower rolling average rates in the summer months. 

Using the actual monthly summer-winter monthly data (rather than the 12-month rolling average CO2 rate) 

finds that 184 units (60% of 306 with data in both time periods) had lower rates in the winter months. This 

supports the argument that operating in higher ambient temperatures tends to degrade efficiency. Looking at 

individual cases, two of the four units with the highest percentage difference in actual CO2 rate, i.e., 2-4 % 

higher in summer months, had exceedances. However, those units are the exception. Over 93 percent of the 

units with higher actual monthly CO2 rates in summer still met the standard with no exceedances. For the large 

majority of post-2000 NGCC units, the 12-month rolling average CO2 rate avoids exceedances by smoothing 

out any increases in CO2 rate that may occur due to ambient temperature. 

Pollution Controls 

                                                           

11 Five‐year capacity factors include only years with MW generation reported.
All the units in this study are assumed to be equipped with low NOX burners. Nearly 90 percent (274 of 307) are 

also equipped with selective catalytic reduction (SCR) NOX controls.12 We found no correlation between units 

with SCRs and average CO2 rate. In fact, five of the 23 units with exceedances did not have an SCR during the 

study period. 

Cycling 

NGCC units can cycle frequently. An examination of starts, defined here as the initiation of fuel consumption, 

found that the average unit started 106 times per calendar year over the study period. Several units in this study 

cycled over 300 times per calendar year, i.e. the equivalent of starting up every weekday. NGCC units 

demonstrate that they are good at cycling efficiently. There is no correlation between the number of starts and 

exceedances. Figure 2 plots the count of starts 2007-2011 (on the X axis) against the average CO2 rate for each 

unit (on the Y axis). The CO2 rates are evenly distributed around the mean and show limited correlation to the 

starts which range from 39 to 1,553. No attempt was made to account for more efficient units operating more 

consistently (and having less starts and stops) due to the lower incremental operating costs compared to less 

efficient units. Of the top 50 units with the most starts, only three had any exceedances. The average CO2 rate of 

the top ten cycling units, measured by start count over the study period, was 883 lb CO2/MWh, well below the 

performance standards. There is no correlation between unit capacity and the start count, i.e., units in the small 

subcategory had no more or less starts than the large units. The average capacity of the top ten cycling units was 

278 megawatts which underlines that both small and large NGCC units operate to meet dispatch requirements. 

FIGURE 2: Unit Start Ups and Average CO2 Rate 2007-2011

 

                                                           

12 No study units had sulfur dioxide or particulate matter controls.   
Elevation 

The elevation of the units in this study ranges from 1 to 5,276 feet above sea level. There is no correlation 

between the elevation of a study unit and either the number of exceedances or the average CO2 rate. None of the 

units with exceedances is located above 1,500 feet. Fifteen units operated at elevations above 4,200 feet and had 

no exceedances during the study period. The average CO2 rate for the fifteen units at altitudes over 4,000 feet is 

862 lbs CO2/MWh, which would indicate that new units situated at higher elevations can maintain a high 

standard of efficiency. 

Figure 3: Unit Elevation and Average CO2 Rate 2007-2011